1. Field of the Invention
This invention relates to an optical information processing apparatus adapted to effect reproduction of information from an information recording medium by projecting a laser beam issuing from a semiconductor laser diode onto the information recording medium, causing the reflected laser beam to return to the semiconductor laser diode and detecting the variations in the characteristic property of the information recording medium from the variations in the returned laser beam.
2. Description of the Prior Art
Recently, optical information processing apparatuses utilizing various forms of optical discs such as video discs otherwise called "picture-producing records," PCM audio discs and optical memories for business use have been developed. These optical information processing apparatuses are intended to effect reproduction of information, for example, from information recording media such as optical discs by projecting a laser beam or some other beam onto the information recording media and deriving the information in the form of variation in the intensity of the reflected laser beam.
As is well known, a minute electroluminescent semiconductor device on the hundred micron order is commercially available. Early electroluminescent semiconductor devices of this type were not capable of continuously operating without being kept at a very low temperature. Recently available devices, however, can operate continuously at room temperature. U.S. Pat. No. 3,812,477 teaches that such a minute semiconductor laser can be used to read bits of information from a compact-storing memory with a high resolution.
According to the method of the U.S. patent a three mirror laser system is used, which comprises a semiconductor laser and a reflector bearing bits of information, and arranged on the optical axis of the semiconductor laser. With this arrangement a resonator system is composed of the semiconductor laser and the reflector, and the reflectivity of the reflector varies depending on whether it bears a bit of information or not. Thus the oscillation of the laser starts and stops depending on whether the laser beam is directed to a recorded bit of information or not and the existence of a bit of information is, conversely, detected from the state of the laser.
In such a system, the lasing of the laser is also started and stopped by wavelength order changes in the length of the optical path.
Further, part of the present inventors have suggested a small readout apparatus which operates by use of the self-coupling effect of the semiconductor laser element (U.S. Pat. No. 4,190,775).
Recently, although the characteristic properties of semiconductor lasers have been notably improved and the degree of the coherence has been enhanced, they entail a new problem in that their operations as self-coupled optical pickups have become unstable.